The Causes of Salinity Swings and Easy Ways To Fix Them - A Simple Guide

Welcome to Simple Reefs. We are a brand new website designed to take the stress and expense out of starting your very own marine aquarium. We’ve been talking about Salinity a lot, lately. I thought this would be a good opportunity to talk about a common problem in marine aquariums. Namely, The Causes of Salinity Swings and How To Deal With Them.

Salinity Swings Are a Common Problem in Marine Aquariums

Fluctuations of your aquarium’s salinity are an inevitability. Even when following best practices, unavoidable things like evaporation are going to lead to, at the least, minor swings in the concentration of salt in your water.

Sometimes these swings can be pretty minor and easy to fix. Other times, they can be quite drastic, particularly if you forget to regularly check your salinity or don’t check the salinity of freshly mixed water.

One of the most common mistakes made by beginners is topping off evaporation with saltwater rather than freshwater. In fact, I talk all about it in my most recent Mishap Mondays article. It is so common and such an easy mistake to make that you can almost predict that it will be the definite cause of salinity swings in a brand new tank with an inexperienced owner.

That fact, alone, can lead brand new owners to have to deal with a major problem with minimal applied knowledge. The good news is, salinity swings are very common and are pretty easy to fix. You just have to take things slowly and do things the right way. Let’s look into the problem a little closer.

Dealing With Salinity Swings

Stability is key to a happy and healthy reef. We reiterate this all the time. In fact, it is to the point where fixing problems can sometimes be less desirable than just stopping them from getting worse. This point has to be kept in mind when dealing with salinity swings.

People get really caught up in chasing the perfect numbers for their marine aquarium. 5ppm Nitrates, 0.05 Phosphates, 420 calcium, etc etc. Sometimes, the best course of action is to just keep things steady, even if the numbers aren’t perfect.

Your reef is rather capable of adapting and will adjust to some degree of imperfection. Hence why making large changes to fix problems can be massively detrimental. When fixing a problem, we have to go slowly and take our time. Don’t just rush to a large water change as the solution to a problem.

When we aim to fix a salinity swing, the main thing to keep in mind is to take things slowly. Don’t rush, this might take a week or two depending on the severity of the problem. That’s okay, though. The health of the aquarium’s creatures is paramount and they are going to need time to adapt to the changes. Before we get into fixes, let’s look at some of the causes of salinity swings.

The Causes of Salinity Swings

Let’s take a look at some of the causes of these swings and changes to salinity. Some of these things are obvious and to be expected while others are down to user or equipment error. It’s easy to make mistakes and you shouldn’t feel bad about it. Just look to correct the problem and learn from the experience. We will look at the solutions to these problems in just a second.

• Evaporation: This is an absolute inevitability. It’s just a fact of life when you have a large body of warm water sitting in the middle of an ambiently warm room. When your water evaporates, the salt will be left behind. This increases the salinity of the water left in your tank as the amount of water will be reduced and the concentration of salt will be higher. This problem can lead to our next cause of evaporation.
• Incorrect Water For Topping Off: This is far more common than many realise. When your aquarium experiences evaporation, you need to top it up. A lot of beginners don’t realise that you actually have to top your tank off with Freshwater, not saltwater. Many top it off with freshly mixed saltwater which will cause your salinity to spike enormously.
• A Faulty ATO: This is another common one. Auto Top Off (ATO) systems automate the process of topping your tank’s evaporation up with freshwater which takes a lot of the hassle out of maintenance. They are prone to failure, though. The sensors can get caked up with calcium, they can get knocked into the water which will stop them working, the pumps can fail. This can easily result in them not working as intended and salinity climbing.
• Mismatched Salinity When Changing Water: This is probably the easiest way to spike or drop salinity. Simply failing to match the salinity of your freshly mixed saltwater to that of your tank’s water. There are a few reasons this can happen that we will discuss in a second.
• Poorly Mixed Salt: This is another common cause of salinity swings, particularly with certain brands of salt. If the salt isn’t thoroughly mixed and dissolved, the salinity measured will appear lower than it is. When you add it to your aquarium, the undissolved salt will then mix and dissolve which will raise the salinity.
• Inaccurate Measurement: If your refractometer is not calibrated or you don’t maintain a consistent temperature (as indicated by the manufacturer) when testing salinity, readings may vary. This can cause inaccuracies in your mixed saltwater as well as the water in your tank.
• Dosing Errors: This is less common but worth considering. When dosing, you sometimes have to consider whether the elements that you are dosing contain additional sodium. In a lot of cases, they do. Dosing shouldn’t result in a rapid swing in salinity but, over time, it can definitely accumulate and may deserve some consideration when you calculate the salinity of your water changes.
• Protein Skimmers: Protein skimmers shouldn’t really cause fluctuations of salinity when working correctly. They should, actually, barely remove any salt at all. You can read all about how protein skimmers work in this article right here. When a protein skimmer is skimming very wet or over skimming, however, this might not be the case. When a protein skimmer runs wet, it will remove more of the saltwater from your tank than is usual. If it is over skimming and you are emptying the cup frequently, this might add up over a week or two.

There are other causes of salinity swings but these tend to be the most common. In fact, I can almost guarantee that you will experience at least one of two of these things. Even if you are exercising an immense level of caution. Let’s take a look at how we can keep ourselves aware of these swings.

Staying Aware To Prevent Salinity Swings

There are ways we can stay on top of fluctuations of salinity. First of all, you want to make checking your salinity a regular thing. I suggest once a week in my water testing schedule but every other day would be even better. Even more frequent testing can only be a benefit if you have a complex reef with delicate coral. It only takes a minute to do, after all.

It’s a very good idea, as a beginner, to get into a habit of testing. Like I say in my article on the importance of frequent testing, turn yourself into a scientist. Grab a notebook or diary, grab a pen, perform your tests and write them down for future reference. You will be grateful of your diligence when it comes to fixing problems.

If you become aware of a swing in your salinity, identifying the cause would be the next best thing to do. Even with perfect husbandry, salinity swings can occur. It is just one of those things that comes along with the imperfect nature of marine aquarium ownership.

Salinity swings can happen gradually over days and weeks or suddenly due to more drastic external factors. Gradual changes aren’t actually as detrimental to your aquarium’s residents as you might think. Changes that take place over days and weeks will actually afford your fish and coral a chance to adapt. While not ideal, it isn’t catastrophic.

Sudden changes, however, can be just that. A big swing from, say, 35ppt (1.026SG) to a much lower number can cause massive stress to your tank and even result in the death of more sensitive coral. Let’s look at how to deal with or prevent salinity swings in your marine aquarium.

Evaporation Causes Salinity to Change

Your aquarium is going to experience evaporation. It’s just one of those things. After all, you are keeping a tank of warm water in a room with multiple ways for the water to escape.

This means you will need to top your tank up with freshwater when you notice this evaporation. This will bring the salinity levels back to what they were prior to the evaporation. Be careful not to top up too much freshwater and not to top up too little.

It’s a good idea to put some kind of mark on your aquarium to note where you tank’s normal water level should be. That way, when the water evaporates, you will have a visual reference point to see where you need to top up to. Obviously, if you have a sump, the mark will need to go in the return area of your sump. On all in one tanks, the mark should go in the filtration chambers.

This task can be automated with an Auto Top Off (ATO) system if you fancy splashing out. The auto top up will monitor your water level and then add fresh water when it gets low for increased stability. All you have to do is add fresh water to the ATO’s water reservoir. It’s super simple and one of the better ways to spend money on a large aquarium.

Auto Top Offs do need some maintenance. Keep the sensor clean so it doesn’t get blocked by salt creep. You can see salt creep on the image of my sensor above. I need to clean it but figured I would get the photo first to illustrate the problem.

Make sure to give the pump a clean every now and then. You will want to clean the freshwater reservoir every now and then, as well. It pays to check whether the ATO is doing its job a couple of times a week, as well. Mine has failed a few times for no good reason and needed fixing.

Remember: Never top up evaporation with saltwater. This is a really easy mistake to make and loads of people do it. Top up evaporation with freshwater. The salt is left behind when the water evaporates so topping up with saltwater will actually increase the salinity significantly.

Thoroughly Mix and Test Your Saltwater

Another way to maintain stability is to be rigorous when mixing your salt water for water changes. Warm up the fresh water to around 25∘C/77∘F, add your salt, and mix thoroughly. A wavemaker or powerhead in the bucket will save a lot of time and achy arms.

Unless your salt mixture suggests otherwise, leave the water for 24 hours to stabilise. Check the salinity of both your tank and your newly mixed water before adding it to the tank and make sure they match. If they don’t, either add more salt or add more fresh water to compensate.

It is easier to add more salt than it is to add more freshwater to an already full bucket. With this in mind, it is always best to add salt gradually and keep checking your salinity. Adding the salt gradually will allow it to mix thoroughly, as well.

Fixing High Salinity

Fixing high salinity is quite easy, the key is to take things slowly. Don’t rush to fix the problem in one day, you may shock your aquarium’s residents. Set a task to fix it over a week or two. All you need to do is remove some of the saltwater from the tank and replace it with an equal amount of freshwater.

A single percent of your tank’s full volume should be fine. Say for example you have a 250 litre aquarium, remove 2.5 litres of saltwater and replace it with an equivalent amount of freshwater every day. You really don’t want to be adjusting the salinity by any more than between 0.001-0.002 SG per day. Particularly if you keep delicate coral.

Keep checking the salinity of your aquarium and do this until your salinity levels are back to where you want them. This slow approach is the safest way to do things and should avoid a big salinity swing.

Fixing Low Salinity

For low salinity, you have a couple of options, the key, once again, is to take things slow. The easiest way to do things is to stop topping up your evaporation with fresh water and top up with saltwater at your desired salinity instead. As the tank evaporates gradually, you will be increasing the salinity gradually, which should avoid shocking your fish or coral.

Another option is to simply perform a typical water change (10-20%) but instead of mixing your water to your intended salinity, mix it a little higher. For example, if you are aiming for 1.025SG but your aquarium’s water is currently 1.022, mix your water change saltwater up to 1.030SG (take a look at our article on what SG and PPT mean and how they are used to measure salinity).

Again, keep the water changes small and you should avoid shocking your residents too much. After a few water changes, your salinity should be back to your desired level and then you can resume your normal routine.

In Short

Salinity swings are a common part of marine aquarium ownership. Frequent monitoring of your salinity can help you get a head start on the problem and even prevent it. Topping up evaporation with freshwater is key to keeping stable salinity. Fixing salinity swings should be done slowly and never all at once to avoid shocking your aquarium’s residents. Thanks for reading and spending your time at Simple Reefs.